By Pat McCool, KRWA Consultant
Feeding Ammonium
Sulfate to Form Combined
Chlorine Residual
M
any Kansas public water supplies (PWSs) add
ammonia to react with free chlorine in the water
to form combined chlorine. Not adding enough
ammonia will result in a decrease (“residual loss”) in
chlorine residual. Adding too much ammonia will result in
waste of chemical and more “food” for nitrifying bacteria.
In calculating the ammonia feed rates, the amounts
of the free chlorine and ammonia reacting to form
monochloramine must be specifically addressed. The
formation of monochloramine has also been written as
follows:
◆ HOC1 + NH4+ g NH2C1 + H3O+ (4)
Ammonia addition
Free chlorine residual is measured with testing equipment
and the results are reported as mg/l of Cl2. That is, the
residual is measured and reported as free chlorine in the
water is in the Cl2 form, which it is not. Free chlorine in the
water is in the forms of HOCl and OCL-, the proportion of
each depends on the pH of the water.
Ammonia is added for two reasons. First, ammonia is
added to stop the reactions of free chlorine that form
trihalomethanes (THMs) and haloacetic acids (HAAs).
Ammonia is added to change the free chlorine residual to
combined chlorine residual because combined chlorine
does not form THMs and HAAs.
Second, ammonia is added to maintain combined
chlorine residual in the distribution system. Ammonia is
added at treatment plants, at wells, and at rechlorination
locations in the distribution systems.
There are mainly three different ammonia chemicals that
can be added. Most PWSs serving less than 15,000 persons
add ammonium sulfate. Larger systems add either
ammonium hydroxide (“aqua ammonia”) or ammonia gas.
Ammonium sulfate is chosen because it avoids safety
issues, some feeding problems, and possibly larger capital
costs. Aqua ammonia and ammonia gas are cheaper
chemicals but the cost savings for small systems are
insignificant when compared to the advantages of using
ammonium sulfate.
Free chlorine and ammonia reactions
In water the possible reactions of free chlorine (HOCl)
and ammonia (NH3) can be represented by the following:
◆ HOCl + NH3 g NH2Cl (monochloramine) + H2O (1)
◆ NH2Cl + HOCl g HNCl2 (dichloramine) + H2O (2)
◆ NHCl2 + HOCl g NCl3 (trichloramine) + H2O (3)
In water treatment it is important to add ammonia
according to the first reaction to form monochloramine.
The last two reactions forming dichloramine and
trichloramine (also called nitrogen trichloride) are to be
avoided because these two reactions also result in a
decrease in chlorine residual.
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THE KANSAS LIFELINE
March 2016
This photo shows ammonium sulfate solution feed equipment
for a 500 GPM water treatment plant. The solution is made
from purchased granular ammonium sulfate in 25 pound bags.
The solution strength is determined by the plant operators.
Definitions of “Ammonia”
The “chlorine-toammonia ratio”
unfortunately has
different definitions to
different operators and
thus different numerical
values or “ratios”.
When discussing or addressing
“ammonia” it is very important to
be knowledgeable and specific as
to which of the three ways
“ammonia” is meant. The three
possible ways are:
1) ammonia (NH3); or
2) ammonium ion (NH4+); or
3) ammonia-nitrogen (NH4-N).
Ammonia and ammonium ion
can be used interchangeably
because their molecular weights are
very close at 17 and 18, respectively. However, the
molecular weight of ammonia-nitrogen (that is, the
nitrogen in ammonia or in ammonium ion) is 14 and that
is a substantial difference with the other two ways of
reporting “ammonia”.
added) to the ammonia (calculated as
ammonia and not ammonia-nitrogen)
added to the water to form
monochloramine without providing
excess, “free”, un-reacted ammonia in
the water.
When using NH3 in reaction (1) or
NH4+ in reaction (4), the ratio is 3.9 or
4.2, respectively, or approximately 4.1.
If ammonia-nitrogen is used as the
basis of “ammonia”, then the ratio is
approximately 5.1.
“Chlorine-to-ammonia ratio”
The “chlorine-to-ammonia ratio” unfortunately has
different definitions to different operators and thus
different numerical values or “ratios”. This causes
confusion and misunderstandings in determining
ammonia dosages. The “chlorine-to-ammonia ratio”
should address the matter of chlorine and ammonia
reacting as in reactions (1) or (4) so as to form
monochloramine.
The following definition is used by many and KRWA.
It is: The “chlorine-to-ammonia ratio” is the ratio of the
free chlorine (in the water just before the ammonia is
This ammonium sulfate solution feeding equipment is used in a
1,000 GPM water treatment plant. The plant purchases the
solution already mixed.
THE KANSAS LIFELINE
March 2016
25
Equation and definitions
Feeding Ammonium
Sulfate Solution
KRWA has worked with operators at
many locations in setting up ammonia
addition, in trouble-shooting operational
problems, in optimizing ammonia addition,
and in correcting residual loss. The
equation KRWA uses when adding
ammonia sulfate solution is explained in the
sidebar at right.
The equation has worked in all situations
except initially in a few situations that have
special circumstances that cause the
formation of monochloramine to be
“inefficient”. Such situations include poor
mixing of chlorine or ammonia in the water,
over-chlorination downstream of ammonia
addition, and too much ammonia addition
due to automatic controls.
Also, it is beneficial to add more
ammonia (approximately
0.1 – 0.3 mg/l) than necessary for
monochloramine formation. The additional
“free”, un-reacted ammonia in the water
results in a (chlorine and ammonia reaction)
system that is more “forgiving”. That is, if
the free chlorine residual increases there
will be ammonia for reaction and the
residual will not decrease.
gph =
(mg/l) x (GPM) x 1.9
(#/gal) x 1,000
gph = gallons per hour (pumping rate of ammonium sulfate solution feed
pump)
GPM = gallons per minute (water flow to which ammonia is being added)
mg/l = ammonia dosage in milligrams of ammonia per liter (ammonia as
NH4+ or NH3)
#/gal = pounds of ammonium sulfate per gallon (ammonium sulfate solution)
1.9 is a conversion factor
1,000 is a conversion factor
These two conversion factors are chosen for simplicity and are a
combination of the following four factors: molecular weights ratio, grams per
pound, minutes per hour, and liters per gallons.
The equation is used for determining ammonium sulfate solution pump
sizes and feed rates; sizing solution feed tanks; determining dosages;
estimating “free” ammonia; determining solution mixing rates; and other
pertinent operational and quality control factors.
For small systems that purchase granular ammonium sulfate in bags and
make solutions, the (#/gal) is usually in the range of 0.25 to 1.5 depending on
solution pump sizing and GPM being treated. If commercial 38 percent
ammonia sulfate solution is being used, check with supplier for (#/gal) or use
3.9 if information is unavailable.
For most all systems, the (mg/l) dosage is usually in the range of 0.5 to 1.5
depending on chlorine residual goal and desired “free” ammonia levels.
When calibrating solution pumps it should be remember that 1.0 gph =
63.08 ml/min (milliliters per minute) or 63 ml/min.
Rechlorination
Many PWSs are rechlorinating water in the
distribution system that has combined chlorine
residual. As shown in the chlorine and
ammonia reactions, to form monochloramine
there must be enough ammonia in the water to
react with the chlorine. Otherwise, the “excess”
chlorine will react with monochloramine in
reaction (2) and chlorine residual will decrease.
Thus, ammonia addition at rechlorination
locations is needed when “free”, un-reacted
ammonia in the water is insufficient for
rechlorination.
Ammonia addition
“Forget-Me-Nots”
Some chemistry, knowledge, and operator
experience are necessary to add ammonia in
correct amounts. However, if there is one
important fact that all operators must
remember, it is this: When forming and
maintaining combined chlorine residuals,
sometimes the correction of chlorine “loss” /
decrease involves doing what is the opposite of
what is intuitively or logically thought to be the
correction.
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THE KANSAS LIFELINE
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For instance, in some situations where chlorine
residual is decreasing, the solution could be to
decrease the chlorine dosage or increase the ammonia
dosage. Another situation, increasing chlorine dosages
may make chlorine residuals decrease. However, all
instances can be explained by the knowledge of the
chlorine and ammonia reactions.
The most frustrating problem encountered when
adding ammonia is the decrease of chlorine residual
or as many say, chlorine “loss”. The most common
correction is adding the correct amount of ammonia
and that usually is more ammonia than is presently
being added.
If any reader believes his/her water system may
have such problems, please contact KRWA as KRWA
staff are available to discuss or help troubleshoot and
correct chlorine and ammonia dosages.
In the July 2016 issue of The Kansas Lifeline, I will
present actual examples concerning use of the
equation to determine ammonia dosages and other
suggestions how to monitor, evaluate, and adjust
dosages.
This ammonium sulfate feed system is for a very small 60 GPM flow
at a rechlorination system at a pump house in a distribution system.
Ammonium sulfate is purchased in 25 pound bags and mixed at only
0.25 pounds per gallon of feed solution. Rechlorination and
ammonia addition are only needed in this system the summer and
early fall months. The chemistry and the chemical reactions are the
same regardless of the size of the feed systems.
Pat McCool has worked as a consultant to
KRWA since January 2004. He previously
worked for KDHE for 30 years. Pat has a
bachelor degree in Chemical Engineering
and a masters degree in Environmental
Engineering from the University of Kansas. THE KANSAS LIFELINE
March 2016
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